According to the Chantry, it is the hubris of men that brought the darkspawn upon the world. They teach that when the magisters of the old Tevinter Imperium entered the Golden City, their sin brought corruption to it. They were cast out, transformed into the hideous monsters known as darkspawn, and forced to flee the light of the sun, retreating into the Deep Roads.
The dwarves give little credit to Chantry beliefs, but they themselves have no known origin story for the darkspawn. As far as the dwarves are concerned, the darkspawn simply appeared. The darkspawn hordes that invaded the Deep Roads ended up crippling the dwarven empire, leaving only Orzammar and Kal-Sharok, the latter of which was completely isolated from the outside world until recently.
Whether the Chantry's information on the origin of the darkspawn is correct or not is uncertain, but no opposing story of their origin has been put forward.
With the fall of the Golden City and the beginnings of the First Blight in 800 TE, a dark age descended onto Thedas as the darkspawn rampaged across the continent, destroying everything in their path. The Tevinter Imperium suffered greatly, losing considerable territory and fracturing under the strain of the invasion, while the subterranean empire of the dwarves was laid waste, with only Orzammar and Kal-Sharok surviving. After nearly a century of hopeless, unending war, the Order of the Grey Wardens was established at Weisshaupt Fortress in 890 TE, and proved instrumental in the defeat of the Archdemon Dumat at the Battle of the Silent Plains. The darkspawn invasion had been beaten, but the species would retreat into the Deep Roads to rebuild and recover. Over the centuries, four more Blights would occur, the latest occurring in 9:30 Dragon (the events of Dragon Age: Origins). Each time, the Blight would be stopped with heavy losses, with the darkspawn destroying entire cities or even kingdoms before being beaten back by the nations of Thedas and the Grey Wardens.
Society and Culture
Unlike the other major races of Thedas, the darkspawn have very little resembling any form of culture or social organization, and can barely be considered sapient. The only hint at a native culture is their propensity to convert statues into representations of the Old Gods. The average darkspawn possesses no intelligence beyond a simple animal cunning, and they do not possess any language beyond roars and grunts. They are also completely hostile to outsiders; anyone caught venturing into the Deep Roads will be attacked, and anything caught in their path during a raid or a Blight will be mercilessly slaughtered. The only exceptions are those infected with the darkspawn taint; it is rumoured that their weapons and armour are constructed by groups of servile ghoul, as the spawn themselves have no capacity to use tools beyond weapons. When not engaged in raids, the darkspawn endlessly search for the sleeping Old Gods, drawn by what they hear as the "song" of the dragons. Should they find one, the taint will transform it into an Archdemon, a guiding intelligence that sends the darkspawn pouring to the surface in a Blight.
The darkspawn horde is connected through the taint, functioning as a hive-mind similar to ants. While high ranking darkspawn (like Emissaries or Alphas) have a limited influence upon small groups, and Architect and The Mother also command larger groups yet, only an Archdemon can command the entire horde.
Territory conquered by the darkspawn becomes diseased, a rotting place of twisted creatures infected by the blight. Creatures that come in close or frequent contact with the darkspawn often become diseased themselves, becoming ghouls, e.g., Blight Wolves or Bereskarns. While the blighted creatures often work alongside the darkspawn, most often because of force, their lifespan is such that their use is severely limited. Even a ghoul's lifespan can usually be measured in months.
It seems that the taint suppresses higher function over time, causing those infected to go mad and potentially cannibalistic. It also allows the tainted to feel the thoughts of the Old Gods. The Joining ritual dramatically slows the corruption of the taint in Grey Wardens, but after about thirty years the major effects of corruption set in. When a Grey Warden begins to hear the call of the Old Gods, they set off into the deep roads to fight to the death before their mind is overtaken. The darkspawn find the Old Gods' call pleasurable, compelling them to seek out the Old Gods. Because the Darkspawn are creatures born of the taint, this is likely the reason why they are incapable of higher thought.
The darkspawn taint also carries magical power equivalent to lyrium. In the Warden's Keep DLC, Avernus reveals that through his research, he has discovered a way to use the taint in Grey Warden blood to cast powerful magic without the use of lyrium or spirits. Even non-mage Wardens can access this power, since all Wardens carry the darkspawn taint. Since the taint in the blood is accessed directly for the magical power, and the blood is not used as a sacrifice for demons, according to certain definitions it is not actually "blood magic".
As a species, the darkspawn are asexual, with the majority, if not all, of their kind being unable to reproduce. This task is left to a specific form of ghoul, referred to as Broodmothers. The broodmothers are prisoners, captured from surface raids and incursions into the Deep Roads, and while male prisoners eventually die, females face a fate far worse. Infected by the taint and forced to consume the flesh of their fellows, those that survive this process undergo a horrific transformation and become broodmothers, massively swollen creatures that exist to breed more darkspawn. A single broodmother is capable of giving birth to thousands, if not tens of thousands of darkspawn during their lives, and each race produces a different type of darkspawn. Humans produce Hurlocks, dwarves produce Genlocks, elves produce Shrieks, and Qunari produce Ogres. Darkspawn sometimes have features similar to the race their broodmother once was, e.g., shrieks have similar ears to elves, hurlocks and genlocks resemble humans and dwarves, respectively, in size, and ogres, like Qunari, have horns. Newborn darkspawn look more or less like the Children.
Role in the series
The Darkspawn reprise their roles from above, but it has been currently revealed that Chernabog was responsible for the creation of the creatures from the darkness in people's hearts in the first place, somewhat making the Darkspawn and Shadow Demons of the voodoo masks the very first Heartless to be created. When Chernabog began his dark conquest, he needed elite generals to control various subsections of the Darkspawn. So he gathered all the various gods and demons of darkness and made them the generals of the Darkspawn, giving rise to the term 'Darkspawn Lord', much the relationship between the Sith aliens and the Dark Lords of the Sith. They currently serve as Malefor and Teridax's goons in the Banished Realms.